Security printing liquid and method using nanoparticles

ABSTRACT

The invention relates to a printing method according to which, during the printing process, one or more narrow nozzles eject a printing liquid, and to a printing liquid suitable for such a method. The invention is particularly suitable for forgery-proof printing on papers or articles. According to the invention, the printing liquid contains nanoparticles that can be induced to fluoresce or phosphoresce. Said nanoparticles are small crystalline particles that can be induced to fluoresce or phosphoresce on their own or when mixed with dopants. Individual dots ( 10, 12 ) can be printed by means of a printing liquid that contains said nanoparticles. Due to their small size of from 1 to 1000 nanometers, preferably in the range of 300 nanometers or even much smaller depending on nozzle diameter, there is no risk of very narrow ink jet nozzles getting plugged. The induction and fluorescence emission frequency ranges are narrow-band so that for a security control of the print the respective wavelength-specific induction or detection is required, thereby increasing protection against forgery.

The invention is based on a printing method according to which, duringthe printing process, one or more narrow nozzles eject a printingliquid, and on a printing liquid suitable for such a method. In thiscase, narrow means a nozzle opening diameter of a few microns. Theinvention relates in particular to the aspect of forgery-proof printing.

In many sectors there is a great interest in securing prints againstforged copies. Particular relevance is exhibited in the production ofcurrency notes, stocks, checks and other papers which represent value orof identity and other identification documents. However, imprints whichincrease security against forgery are also applied to an increasingextent in the identification verifying the authenticity of products,such as that of CDs, computer chips or drugs.

One known method of protecting against forgeries is the use of graphicsecurity features which, because of their inconspicuousness ormicrostructure, are not perceived by the naked eye. Thus, with themethod disclosed in patent specification DE 197 54 776 A1, very smallstructures with particularly fine lines of up to 5 microns width can beprinted.

In addition, in the method disclosed in patent specification DE 199 00856 C2, additional secondary information which cannot be detected by thenaked eye is further introduced into the visible image to be printed,such as, for example, by changing the shape, the density, the positionor the size of the image points.

However, such methods have the disadvantage that the protection againstforgeries merely consists in the detectability and reproducibility beingmade more difficult by particularly small and possibly at the same timehidden structures in the print. Procuring or producing the necessary,particularly high-resolution special printers and their drive meansrepresents a high technical and financial outlay. Nevertheless, withsimple optical enlargement means, such as a magnifying glass, thissecondary information can be discovered quickly and forged duplicatescan be prepared with appropriately high-resolution printers.

Furthermore, printing methods and printing liquids are known in theprior art in which the security against forgery is increased by the useof organic dyes which can be induced to fluoresce. However, such methodsand printing liquids have the disadvantage that organic dyes have both abroadband absorption spectrum and a broadband emission spectrum. A sharpdelimitation of the absorbable and emitted radiation frequencies, whichis necessary for accurate identification, is not possible. Consequently,these methods and printing liquids are inadequate for protection againstforgeries.

Likewise inadequate for printing by means of narrow nozzles are knownprinting methods and printing liquids in which the printing liquidcontains micro particles which can be induced to fluoresce and are ofthe order of magnitude of a few microns. This is because, in the case ofnarrow nozzles, whose diameter can be down to about 5 microns small,there is a great probability of being blocked quickly by the microparticles.

It is therefore an object of the present invention to provide a printingmethod according to which, during the printing process, one or morenarrow nozzles eject a printing liquid, and a printing liquid suitablefor such a method, with improved security against forgery with respectto printing.

ADVANTAGES OF THE INVENTION

The subject of the independent claims achieves this object. The printingliquid according to the invention contains nanoparticles which can beinduced to fluoresce or phosphoresce and which, according to thewording, are particles of the order of magnitude of 1 to 1000 nanometersdiameter and have a crystal structure. In the case of such smallparticles, there is no risk of blockage of the narrow nozzles used inthe printing process, in particular if they have smaller averagediameters than 300 nanometers. The use of printing liquids with suchnanoparticles is a simple method of increasing the security againstforgery of prints, in particular since, in the case of such crystallinesolid bodies which can be induced to fluoresce or phosphoresce, there isa sufficiently sharp delimitation of the absorbable and emittedradiation frequencies for improved forgery-proof identification.Depending on the intended application, the nanoparticles can be excitedwith UV-A, UV-B or UV-C radiation or with visible light.

The present invention ensures an improvement in security in productprotection by marking objects by means of printing liquid containingnanoparticles, in particular one which can be printed by inkjet.

Advantageous developments and improvements of the respective subject ofthe invention will be found in the subclaims.

With the possibility of printing nanoparticles with an inkjet printer,new possibilities in product protection are provided. In order to createa sufficient distance from a possible forger, use should be made of thefact that a print with an inkjet printer is a dot print. The lines whichare visible to the eye consist of a row of dots, a method ofrepresentation which is uncommon in other printing methods. Ifnanoparticles are then used in a multicolor printing process, it becomesmore and more difficult to imitate the dot pattern which is produced.Thus, it is possible to cause an image point to be produced from threedifferent individual dots, of which one is a dot specifically markedwith nanoparticles.

The detection of such a method should preferably be extremely simple andentail little expenditure on apparatus.

If, for such a security print, use is made for example of a colorlessink containing lanthanum phosphate nanoparticles, and doped with ceriumand terbium (LaPo4:Ce;Tb) and two inks each having another color in amulticolor printing method, such as is the case nowadays in commerciallyavailable desktop printers, then it is possible to incorporate in aprint a security feature which only becomes visible under a UV-C (255nm) lamp but is not visible under a UV-B (366 nm) lamp.

This means that this print cannot be imitated with organic dyes, sinceorganic fluorescent dyes luminesce under both light sources. A forgerywith micro particles—whether organic or inorganic—is likewise preventedin this case, since micro particles cannot be printed by an inkjetmethod—or generally by a printing method in which the printing ink isejected through narrow nozzles. Using other printing methods, it is notpossible to imitate readily the dot pattern produced in the inkjetprinter. Using the known piezo printing method, it is even possible toproduce specific printing patterns, depending on the design of the printhead.

In summary, the security of the marking is guaranteed by the size andthe physical peculiarities of the nanoparticles and by the use of thesame in an ink in a multicolor printing method of an inkjet printer.

The present invention is therefore suitable in particular forintroducing fluorescent or phosphorescent nanoparticle substances inliquids suitable for printing as a carrier medium, to mix the latterwell therein and therefore to carry out a printing method according tothe prior art in the form modified in accordance with the invention, orto improve the printing method further with regard to the production ofprinted security markings, as has been described further above.

In this case, use can be made in particular of those nanoparticles whosesynthesis is disclosed in the simultaneously pending patent applicationPCT/DE01/03433.

These are substantially metal salt nanoparticles with a crystal latticeor whorl lattice, whose cation can be obtained from a cation source andwhose anion can be obtained from a material class used as an anionsource, it being possible for the whorl or lattice material to contain,in particular, compounds from the group of phosphates, halophosphates,arsenates, sulfates, borates, aluminates, gallates, silicates,germanates, oxides, vanadates, niobates, tantalates, tungstates,molybdates, alkali halides, other halides, nitrides, sulfides,selenides, sulfoselenides and oxysulfides. A dopant or more which maypossibly be present can then be selected specifically such that therespectively desired absorption and emission properties can beimplemented.

In principle, in the form when nanoparticles are added to one or morecolored liquids, the method according to the invention can of coursealso be applied to achieve “simple” special fluorescence effects. Thisis substantially to be understood to include all those effects whicharise on account of a fluorescent emission which can be perceived in anuncomplicated manner, specifically following excitation with radiationfrom a spectral range which is common and can be produced simply, evenrelatively broad, for example visible light or UV-A. In this case, theluminous effect should therefore be visible easily and without furthertechnical aids. For this purpose, those nanoparticles withoutcorresponding security doping, above all those containing phosphorus orfluorine, are then suitable.

The security aspect of the printing method of the present invention isbrought about substantially by one or more dopants being added, that isto say incorporated in the whorl material of the nanoparticles, at leastone, following appropriate excitation, for example by means of UV-Clight, then ensuring an emission which can then be detected. Thisprinciple is therefore based on wavelength-specific energy absorptionand on wavelength-specific emission of radiation to be detected.

The crystal lattice or, in the case of doping, the whorl lattice, cancontain, expressed in general form, compounds of the type XY, X being acation from one or more elements from the main groups 1 a, 2 a, 3 a, 4a, from the secondary groups 2 b, 3 b, 4 b, 5 b, 6 b, 7 b or thelanthanides (element group of the rare earths) of the periodic system,and Y being a multi-atom anion from one or more elements from the maingroups 3 a, 4 a, 5 a, from the secondary groups 3 b, 4 b, 5 b, 6 b, 7 band/or 8 b and/or elements from the main groups 6 a and/or 7, or asingle-atom anion from the main group 5 a, 6 a or 7 a of the periodicsystem.

These are, in particular: phosphates, halophosphates, arsenates,sulfates, borates, aluminates, gallates, silicates, germanates, oxides,vanadates, niobates, tantalates, tungstates, molybdates, alkali halides,other halides, nitrides, sulfides, selenides, sulfoselenides or theoxysulfides.

The dopant used can be one or more elements from a set containingelements from the main groups 1 a, 2 a or Al, Cr, Tl, Mn, Ag, Cu, As,Nb, Ni, Ti, In, Sb, Ga, Si, Pb, Bi, Zn, Co and/or lanthanide elements.

Amongst others, nanoparticles with one of the following compound can beused for marking. In each case the substance used for doping is notedafter the colon: LiI:Eu; NaI:Tl; CsI:Tl; CsI:Na; LiF:Mg; LiF:Mg,Ti;LiF:Mg,Na; KMgF₃:Mn; Al₂O₃:Eu; BaFCl:Eu; BaFCl:Sm; BaFBr:Eu;BaFCl_(0.5)Br_(0.5):Sm; BaY₂F₈:A(A=Pr, Tm, Er, Ce); BaSi₂O₅:Pb;BaMg₂Al₁₆O₂₇:Eu; BaMgAl₁₄O₂₃:Eu; BaMgAl₁₀O₁₇:Eu; (Ba, Mg)Al₂O₄:Eu; Ba₂P₂0 ₇:Ti; (Ba, Zn, Mg)₃Si₂O₇:Pb; Ce(Mg, Ba) Al₁₁O₁₉;Ce_(0.65)Tb_(0.35)MgAl₁₁O₁₉; MgAl₁₁O₁₉: Ce,Tb; MgF₂:Mn; MgS:Eu; MgS:Ce;MgS:Sm; MgS(Sm, Ce); (Mg, Ca)S:Eu; MgSiO₃:Mn; 3.5MgO.0,5MgF₂.GeO₂:Mn;MgWO₄:Sm; MgWO₄:Pb; 6MgO.As₂O₅:Mn; (Zn, Mg)F₂:Mn; (Zn, Be)SO₄:Mn;Zn₂SiO₄:Mn; Zn₂SiO₄:Mn,As; ZnO:Zn; ZnO:Zn, Si, Ga; Zn3(PO₄)₂:Mn;ZnS:A(A=Ag, Al, Cu); (Zn, Cd)S:A(A=Cu, Al, Ag, Ni); CdBO₄:Mn; CaF₂:Mn;CaF₂:Dy; CaS:A(A=lanthanide, Bi); (Ca, Sr)S:Bi; CaWO₄:Pb; CaWO₄:Sm;CaSo₄:A(A=Mn, lanthanide); 3Ca₃(PO₄)₂.Ca(F, Cl)₂:Sb, Mn; CaSiO₃:Mn,Pb;Ca₂Al₂Si₂O₇:Ce; (CA, Mg)SiO₃:Ce; (Ca, Mg)SiO₃:Ti; 2SrO.6(B₂O₃).SrF₂:Eu;3Sr₃(PO₄)₂.CaCl₂:Eu; A₃(PO₄)₂.ACl₂:Eu(A=Sr, Ca, Ba); (Sr, Mg)₂P₂O₇:Eu;(Sr, Mg)₃(PO₄)₂:Sn; SrS;Ce; SrS;Sm,Ce; SrS:Sm; SrS:Eu; SrS:Eu,Sm;SrS:Cu,Ag; Sr₂P₂O₇:Sn; Sr₂P₂O₇:Eu; Sr₄Al₁₄O₂₅:Eu;SrGa₂S₄:A(A=lanthanide, Pb); SrGa₂S₄:Pb; Sr₃Gd₂Si₆O₁₈:Pb,Mn; YF₃:Yb,Er;YF₃:Ln(Ln=lanthanide); YLiF₄:Ln(Ln=lanthanide); Y₃Al₅O₁₂:Ln(Ln=lanthanide); YAl₃(BO₄)₃:Nd,Yb; (Y,Ga)BO₃:Eu; (Y,Gd)BO₃:Eu;Y₂Al₃Ga₂O₁₂:Tb; Y₂SiO₅:Ln (Ln=lanthanide); Y₂O₃:Ln(Ln=lanthanide);Y₂O₂S:Ln (Ln=lanthanide); YVO₄:A(A=lanthanide, In); Y(P,V)O₄:Eu;YTaO₄:Nb; YAlO₃:A(A=Pr, Tm, Er, Ce); YOCl:Yb,Er;LnPO₄:Ce,Tb(Ln=lanthanide or mixtures of lanthanides); LuVO₄:Eu;GdVO₄:Eu; Gd₂O₂S:Tb; GdMgB₅O₁₀:Ce,Tb; LaOBrTb; La₂O₂S:Tb; LaF₃:Nd,Ce;BaYb₂F₈:Eu; NaYF₄:Yb,Er; NaGdF₄:Yb,Er; NaLaF₄:Yb,Er; LaF₃:Yb,Er,Tm;BaYF₅:Yb,Er; Ga₂O₃:Dy; GaN:A(A=Pr, Eu, Er, Tm); Bi₄Ge₃O₁₂; LiNbO₃:Nd,Yb;LiNbO₃:Er; LiCaAlF₆:Ce; LiSrAlF₆:Ce; LiLuF₄:A(A=Pr, Tm, Er, Ce);GD₃Ga₅O₁₂:Tb; GD₃Ga₅O₁₂:Eu; Li₂B₄O₇:Mn,SiO_(x):Er,Al (0<x<2).

In an advantageous way, nanoparticles with one of the following compoundcan be used according to the invention, since, as is known, they arewell-suited for fluorescence:

YVO₄:Eu; YVO₄:Sm; YVO₄:Dy; LaPO₄:Eu; LaPO₄:Ce; LaPO₄:Ce,Tb; ZnS:Tb;ZnS:TbF₃; ZnS:Eu; ZnS:EuF₃; Y₂O₃:Eu; Y₂O₂S:Eu; Y₂SiO₅; SiO₂:Dy; SiO₂:Al;Y₂O₃:Tb; CdS:Mn; ZnS:Tb; ZnS:Ag; ZnS:Cu; Ca3(PO₄)₂:Eu²⁺;Ca3(PO₄)₂:Eu²⁺,Mn²⁺; Sr₂SiO₄;:Eu²⁺; or BaAl₂O₄:Eu²⁺.

Or, in addition, the following: MgF₂:Mn; ZnS:Mn; ZnS:Ag; ZnS:Cu;CaSiO₃:A; CaS:A; CaO:A; ZnS:A; Y₂O₃:A or MgF₂:A (A=lanthanides).

As a doping, two elements in a predetermined relative concentration witheach other can advantageously be contained, one doping element having alocal maximum of the absorption spectrum to light, in particular UVlight, and the other doping element having a fluorescence emissionspectrum which has at least one local maximum which is at a distanceΔλ/λ of at least 4% from the absorption maximum of the first dopingelement. The aforementioned lanthanum phosphate with dopings of ceriumand terbium is one example of this, one dopant acting as an energyabsorber, in particular as a UV light absorber and the other as afluorescent light emitter.

Increased difficulty of authenticity testing and therefore increasedsecurity against forgery is provided in the case of a doping which emitswhen it was excited with quite specific, narrowband radiation. Theemission can additionally also be detectable only with technical aids,for example UV emission or IR emission.

It goes without saying that a printing method according to the inventioncan also be achieved by the nanoparticles being mixed with one or aplurality or all of the printing inks used. In the case of a three-colorprint, therefore, for example the red color component could be providedwith an appropriate fluorescent nanoparticle mixture. The higher theproportion of the fluorescent nanoparticles in the total volume, themore intensive is the fluorescence and therefore the easier it is todetect the emitted light.

Alternatively, an image point can be produced, not as usual inpixel-based color printing methods, from three individual points of adifferent color in each case but from a plurality, for example four orfive or a higher number.

In addition, in a further advantageous manner for the production of highsecurity markings, specific print heads can be constructed which have asecret pattern arrangement of pixel arrangements. In this case, thepattern arrangement covers an area of 40×40 dots, for example. It can berepeated or modified specifically, for example in accordance with apredefined code—a secret rule—in order to increase the security againstforgery further.

A further development according to the invention of the printing method,in which the printing liquid(s) is/are sprayed out through a pluralityof narrow nozzles, is the possibility of driving individual nozzles orof subsets of the nozzles with respect to the time duration or intensityof the flow of the printing liquid. For example, by increasing theelectric drive voltage on a single driven nozzle of a piezo print head,the flow of the ink can be increased. As a result, the correspondingpixel point can be represented more boldly in a secret patternarrangement, which represents a further possible variation for asecurity marking and therefore increases the protection againstforgeries.

Depending on the appropriate strategy relating to the utility of thesecurity features on the product to be protected, a route can also beselected in which individual ones of the aforementioned securitymeasures can be combined with one another in order to be able to utilizethe effects in each case present selectively in the individual features.

DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawingsand explained in more detail in the following description.

FIG. 1 shows, schematically, two examples of rows of image points whichhave been printed in accordance with one configuration of the methodaccording to the invention, and

FIG. 2 shows, schematically, an example of a secret arrangement of pixelarrangements which has been printed in accordance with a furtherconfiguration of the method according to the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In the figures, identical reference symbols designate identical orfunctionally identical components.

FIG. 1 shows an example, to be understood only schematically, where, onthe left, a line pointing obliquely upward and, on the right, a linerunning vertically, printed as a detail, are shown. The points 10 and 12shown full—1 of 3 on the left, 1 of 4 on the right—are in each case tobe produced from ink supply containers which are enriched with thepossibly secretly fluorescing nanoparticles. These nanoparticles aresmall crystalline particles which, either themselves or with dopantsadded, can be induced to fluoresce or phosphoresce. Individual dots 10,12 can be printed with printing liquid containing nanoparticles comingfrom a special container. Because of their small size of 1 to 1000nanometers, preferably in the region of 300 nanometers diameter or evenvery much smaller, depending on the nozzle diameter, there is not therisk of blockage of very narrow inkjet nozzles.

One example of a secret pattern arrangement of pixel arrangements of aspecific print head, such as a print head of a piezo printing method, isshown in FIG. 2. In this case, the pattern arrangement covers a regionof 40×40 dots in this example. It can, for example, also be repeated ormodified specifically in accordance with the predefined code—a secretrule—in order to increase the security against forgery further.

Each individual point in the pattern arrangement is now intended tocorrespond to a threefold or fourfold image point, as shown in FIG. 1.The pattern can be present repeatedly in a print head, if appropriaterepeated many times.

Examples which may be mentioned of the increased difficulty of forgingthe marking are:

a print head suitable for printing liquids containing nanoparticles, forexample for a piezo printing method, or an inkjet method in accordancewith the thermally based bubblejet principle, which bears a secret pixelarrangement which is difficult to forge and difficult to detect, or:

the doping of a printing ink component is selected such that thecorresponding nanoparticles can be produced synthetically under onlyextremely difficult conditions.

An example which may be mentioned of the ability to be detected easily,that is to say the ability to verify the authenticity of the marking,is: the emission of the nanoparticles can easily be detected by thelayman, for example the teller in the case of a banknote. This canprovide a contribution to configuring banknotes and other printablevaluable paper articles to be forgery-proof, the detection of a forgerybeing capable of demonstration with relatively simple means, dependingon the doping.

Although the present invention has been described above by using apreferred exemplary embodiment, it is not restricted thereto but can bemodified in many ways.

Finally, the features of the subclaims can be combined with one anothersubstantially freely and not in the order present in the claims,provided they are independent of one another.

1. A printing liquid adapted to be sprayed onto objects and throughnarrow nozzles, wherein the liquid contains nanoparticles which can beinduced to fluoresce or phosphoresce.
 2. The printing liquid as claimedin claim 1, wherein the nanoparticles can be induced to fluoresce orphosphoresce by UV-A, UV-B or UV-C radiation or visible light.
 3. Theprinting liquid as claimed in claim 4, wherein the nanoparticles causethe fluorescent or phosphorescent radiation emitted to lie in aninvisible frequency range.
 4. The printing liquid as claimed in claim 1,wherein the nanoparticles contain dopants of at least one sort with anexcitation frequency range and an emission frequency range forfluorescence or phosphorescence.
 5. The printing liquid as claimed inclaim 4 wherein the excitation frequency range and the emissionfrequency range of the dopants are frequency-shifted.
 6. The printingliquid as claimed in claim 4 wherein at least one sort of the dopantscan be assigned to the element group of the lanthanides.
 7. A printingmethod comprising the step of spraying out a printing liquid through atleast one narrow nozzles wherein the printing liquid is adapted to besprayed onto objects and through narrow nozzles and wherein the liquidcontains nanoparticles which can be induced to fluoresce orphosphoresce.
 8. The printing method as claimed in claim 7, whereinthere is at least one of the printing liquids which is sprayed outthrough a plurality of narrow nozzles, and driving the nozzlesindividually or in subsets for causing each of the nozzles spraying andnot spraying the printing liquid.
 9. The printing method as claimed inclaim 7, wherein the printing liquid is sprayed out through a pluralityof the narrow nozzles, and driving the nozzles individually or insubsets with respect to the time duration or intensity of the flow ofthe printing liquid.
 10. The printing method as claimed in claim 7,wherein the method is a piezo printing method.
 11. The printing methodas claimed in claim 7, wherein the spraying by the nozzles follows asecret rule.
 12. A printing apparatus containing devices forimplementing the printing method as claimed in claim
 7. 13. An objectprinted with a printing liquid as claimed in claim
 1. 14. An objectprinted in accordance with the method as claimed in claim
 7. 15. Theprinting liquid of claim 5, wherein at least one sort of the dopants canbe assigned to the element group of the lanthanides.
 16. The printingliquid of claim 15, wherein the dopants comprise chemical elementsCerium and Terbium.
 17. The printing liquid of claim 16, wherein theprinting liquid contains lanthanide phosphate particles dotted with theCerium and the Terbium.
 18. The printing method as claimed in claim 8,further comprising driving the nozzles individually or in subsets withrespect to the time duration or intensity of the flow of the printingliquid.